In the development of VLSI technology, there is a strong demand for improved microfabrication techniques and materials, e. g., refractory metals, which are used for self-aligned gate processes. Conventionally used polysilicon, although having many desirable properties, such as good etchability, good oxidation characteristics, mechanical stability at high temperatures, excellent step coverage and adhesion, has the major disadvantage of a relatively high resistance. A heavily doped 0.5 micron thick polysilicon film, for example, has a sheet resistance of about 20 to 50 ohms per square, which is a major constraint in VLSI circuit design. Therefore, as line widths in VLSI circuits shrink, the major speed limitations arise from the RC time constant associated with silicon gates and polysilicon interconnect lines, thereby limiting high speed performance at very reduced geometries. To reduce interconnect resistivity, it is desirable to deposit refractory metals or metal silicides instead of polysilicon lines.
Refractory metals for VLSI applications are customarily deposited by three different methods: sputtering, evaporation, and chemical vapor deposition. The main advantage of the sputtering process is that both pure refractory metals and refractory metal silicides can be sputtered. The disadvantage of sputtering is poor step coverage.
Evaporation of refractory metals has been investigated as a means for forming VLSI. However, evaporation has many of the deficiencies associated with sputtering. For example, step coverage is poor, and the deposition process is complex using evaporation techniques.
Chemical vapor deposition (CVD) and low-pressure chemical vapor deposition (LPCVD) of refractory metals offer several advantages over sputtering and evaporation techniques. CVD of refractory metals can provide good coverage, reduced system complexity, and higher purity deposits. Also, in some applications, selective CVD does not require an additional photolithography step when the refractory metal is deposited only on areas with certain chemical reactivities. For example, tungsten hexafluoride will react with silicon or polysilicon gates, but not with the surrounding silicon dioxide isolation areas.
However, tungsten films formed in the past by CVD methods have suffered from a number of limitations. Tungsten films formed by the hydrogen reduction of tungsten hexafluoride, according to the equation, EQU WF.sub.6 +3H.sub.2 +W+6HF (1)
produce hydrofluoric acid as a by-product. This is undesirable since the HF tends to etch away the silicon dioxide area surrounding the polysilicon gate, potentially destroying the device. Also, the thickness of films formed by the hydrogen reduction method is difficult to reproduce, and the films formed by this method are highly stressed which can cause delamination of the films from the substrate.
Tungsten films also have been formed by the silicon reduction of tungsten hexafluoride according to the equation: EQU 2WF.sub.6 +3Si.fwdarw.2W+3SiF.sub.4 (2)
This reaction has two major disadvantages. Like the hydrogen method, the films produced by this method are highly stressed. Furthermore, the silicon reduction method requires that silicon be available in order for the reaction to take place. As the tungsten is deposited, less and less silicon is available from the underlying area, which causes the reaction to be self-limiting. Typically, only films of about 30 to 40 nm thickness can be deposited. Beyond this thickness, other methods of depositing tungsten are required.
A refinement of the CVD method consists in decomposing tungsten hexafluoride by igniting a discharge plasma, which permits a drastic reduction in the reaction temperature. This most up-to-date method is known as Plasma Enhanced Chemical Vapor Deposition (PECVD).
However, the reaction gas tungsten hexafluoride used in CVD and PECVD methods poses several severe problems. Tungsten hexafluoride is highly toxic. Due to its boiling point of 17.06.degree. C. and vapor pressure of 1.6 bar at 30.degree. C., longer lines have to be avoided and/or the temperature of the entire supply means has to be stabilized. Further, tungsten hexafluoride has been found to be difficult to control, and it decomposes valves and flow controllers. And, it is difficult to obtain in a highly pure form, and, in addition, when available in that form, it is very expensive.